Oxide single crystals constitute crucial materials for many electronic and optoelectronic applications, such as lasers, non-linear optics, scintillators, piezoelectrics, transparent semiconductors or transparent conductors, superconductors and like. For many, if not majority of such applications bulk singles crystals are required, from which electronic or optoelectronic components can be prepared, or substrates for thin film depositions. Since oxides have a wide range of melting points, roughly ranging between about 800° C. and about 3000° C., different thermodynamic behavior as well as different thermal and physical properties, a number of growth techniques have been developed to obtain bulk single crystals. Such growth techniques can be divided into methods utilizing crystallization from: (i) the liquid phase (melt), (ii) the vapor phase, and (iii) the solution.
The group (i) includes the Czochralski method, the vertical and horizontal Bridgman method, the Kyropolous method, the Heat Exchanger Method (HEM), the Verneuil method, the Skull Melting (or Cold Crucible) method, the Optical Floating-Zone method (OFZ), the Edge-Defined Film Fed Growth method (EFG), the Micro-Pulling Down method, the Laser-Heated Pedestal Growth method (LHPG), and modifications of these techniques. Melt crystal growth techniques can be divided into methods using a metal crucible (Czochralski, Bridgman, Kyropolous, HEM, EFG, and Micro Pulling Down) and no metal crucible at all (Verneuil, Skull Melting, OFZ, LHPG). The principles of these methods are easily accessible in technical and scientific publications. The present invention refers to a novel method which may be classified under the group (i) using the metal crucible.
The group (ii) includes the Chemical Vapor Transport (CVT), Physical Vapor Transport (PVT) and their modifications. The group (iii) includes the Flux method, Hydrothermal method and Top-Seeded Solution Growth method (TSSG).
There are also a number of thin film or layer growth techniques and they require substrates, on which the films or layers can be deposited. Examples of such techniques include: Spray Pyrolysis and Hydrolysis, Sol-Gel, Sputtering, Electron Beam Evaporation, Pulsed Laser Deposition (PLD), Molecular Beam Epitaxy (MBE), Metal Organic Chemical Vapor Deposition (MOCVD), and like.
Oxides are typically electrical insulators, but there is a group of oxides, which exhibit semiconducting or conducting behavior once in the crystalline state. Since oxides have usually a wide optical band gap (>2 eV), they are also transparent to visible light, contrary to classical semiconductors. Such materials are known as Transparent Conducting or Semiconducting Oxides (TCOs or TSOs). Such TCOs or TCOs include gallium oxide (β-Ga2O3), zinc oxide (ZnO), tin oxide (SnO2), indium oxide (In2O3) and several other materials, such as cadmium oxide (CdO), nickel oxide (NiO) and copper oxide (CuO). Since TCOs and TSOs exhibit both transparency, even down do deep ultraviolet region (DUV), and semiconducting or conducting behavior, they have been or can be used in a wide range of electronic and in particular optoelectronic applications, such as transparent electrodes for solar cells and flat panel displays, energy efficient windows, transparent thin film transistors such as MISFET and MESFET, Schottky diodes, light emitting diodes and gas sensors.
In2O3 doped with Sn (so called ITO) is an important industrial material, which is widely used in the form of amorphous layers as transparent electrodes for solar cells and flat panel displays. Pure (i.e. undoped) In2O3 can potentially be used in all other applications typical for TCOs or TSOs.
In2O3 in the form of very small single crystals has been known for over 50 years. The first single crystals of In2O3 were grown by the flux method (J. P. Remeika, E. G. Spencer; J. Appl. Phys. 35, 1964, p 2803) and later from the vapor phase (J. H. W. De Wit; J. Cryst. Growth 12, 1972, p. 183) and electrolysis (N. Imanaka et al.; J. Cryst. Growth 264, 2004, p. 134). In each case the crystals were of very small size (needle-shaped crystals or very small plates), insufficient for any practical application, and were also contaminated by the solvent or chemical agents.
Also layer growth techniques were applied to pure In2O3 and the resulting In2O3 films were either amorphous or crystalline.
Truly bulk In2O3 single crystals have not been available so far. Lack of bulk single crystals limits above-discussed applications of In2O3, and electrical properties thereof in large single crystals still remain unexplored. The reason, why bulk In2O3 single crystals could not be grown from the melt, lies in the chemical instability of that compound at elevated temperatures, that is, In2O3 starts to decompose just above 1000° C., far below its melting point which is about 1950° C. To stabilize In2O3 high oxygen partial pressure in the growth chamber is required (>1 bar), but this is in contradiction to the uppermost acceptable level (approximately 0.02 bar) for using iridium as a crucible material. For other high-melting refractory crucibles (W, Mo, Re), oxygen is not allowed. Generally, it is impossible to melt and grow In2O3 single crystals in iridium crucibles using state-of-the art melt growth techniques. Indeed there are no reports in scientific and technical literature on bulk In2O3 single crystals grown from the melt by any of the above-described techniques.
Therefore, most, if not all of the In2O3 applications mentioned above are based on thin films or layers, while those application areas, in which In2O3 single crystals would function as “self-standing” components for electronic devices or as substrates for homo- and heteroepitaxy still remain unexplored. E.g. epitaxy of In2O3 is performed on substrates prepared from other crystals, such as Y-stabilized ZrO2, and such heteroepitaxy decreases crystalline perfection of In2O3 (such as dislocations, grain boundaries etc.), which may have a great impact on final device performance and lifetime. The availability of bulk In2O3 single crystals and substrates thereof would increase the range of its applications, especially at the industrial scale as well as improve the final device properties and their lifetime.